The present invention relates to a method for determining, by means of a remote detection device of the pulse-compression radar or sonar type, the speed of a target which is moving toward said remote detection device, as well as to a device for implementing said method.
In the context of the present invention, pulse is understood to mean a group of oscillations capable of being subjected to compression as described below.
For reasons of simplification of the description, the invention will be presented hereinafter solely in relation to a remote detection system of the radar type. However, all the characteristics of the invention which will be presented by reference to such a radar clearly apply in the same way equally to a sonar.
The present invention applies more precisely to pulse-compression radars or sonata which transmit frequency-modulated pulses with a linear and continuous variation law. In a normal way, such pulses transmitted by a radar are picked up after reflection on a target to be analyzed, then are processed. The processing is intended to reduce their duration and to increase their amplitude. These pulses thus processed or "compressed" make it possible very precisely to calculate the distance between said target and said radar. It is thus possible to equip such a pulse-compression radar with a precise rangefinding means exhibiting good range resolving power.
However, the range measurements performed by a radar equipped with such a rangefinding means are falsified when the target is moving with respect to said radar, by reason of a phenomenon of ambiguity between the speed and the range which is characteristic of linear frequency modulation. For example, when the radar transmits a pulse the frequency of which increases linearly over time, a moving target which is traveling away from said radar is located at a greater range than the true range, and conversely when the moving target is approaching said radar. These measurement errors are reversed when the radar transmits a pulse the frequency of which decreases over time.
In a known way, this defect of the pulse-compression radar can be eliminated by modulating the transmitted pulse, with an increasing frequency law during the first half of the transmission duration, then with a decreasing frequency law during the second half of the transmission duration, or conversely. Under these transmission conditions, the pulse picked up and processed makes it possible precisely to calculate the distance between the moving target and the radar.
The object of the present invention is to associate with such a pulse-compression radar, making it possible precisely to determine the range of a target, a means making it possible moreover to determine the speed of said target when the latter is moving toward said radar.
It will be noted that the document U.S. Pat. No. 5 294 932 discloses a method making it possible simultaneously to measure the speed and the range of a moving target by means of a radar. However, this method applies to a radar with high recurrence frequency which is by design, extremely precise for speed measurement and which is particularly well adapted to the airborne-radar surveillance function. The method described in this document makes it possible to combine, with this speed measurement, range measurement which, however, exhibits a limited accuracy.
Consequently, the above-mentioned American document and the present invention apply to different types of radar, both in terms of the frequencies used and in terms of the measurements performed, since the present invention is implemented, as previously indicated, in a pulse-compression radar which operates at low or medium recurrence frequency and which is, by design, very accurate for range measurement. This operating mode is employed, in particular, in remote detection radars.
No means currently exists making it possible to determine the speed of a moving target from such a pulse-compression radar.